A typical method to install cables in ducts includes water is injected under pressure into the duct, exerting a pushing force at a pig attached to the front end of the cable, the pig exerting also a pulling force at the cable. At the same time the cable is pushed into the duct with water under pressure and with a mechanical drive, e.g. caterpillars or wheels, disposed at the rear end of the duct.
One system generally includes blowing a cable with the help of pistons, without sealing rings, which are released from a pressure chamber at the cable entrance point at regular intervals. The pistons are just a little smaller in diameter then the inner diameter of the duct. The force over the piston is generated by the airflow. There is no other mechanism to control the force over the pistons.
Another method generally includes using multiple pulling pigs. Here the intermediate pulling pig gets extra pulling force when the seal got stuck somewhere at the duct route (it does nothing when the duct is smooth). It is not intended here to increase cable installation length by distributing the available pushing force over more locations (and reduce the capstan effect), but to get the cable passing at positions where the cable got stuck. The front pig does not allow the passage of air, essential for functioning of the present invention.
The methods described above are often referred to as 30 push/pull method. The advantage of using water instead of air is its buoyancy, reduces the effective weight of the cable. Also it cools the frictional heat. Furthermore the safety of water under pressure is less a problem than for compressed air. This makes the technique with water under pressure suitable for use with larger cables in larger ducts, e.g. high power electric cables. However, for smaller cables in smaller ducts, e.g. telecom cables (like fiber optic cables) the same method can also be used with compressed air (still called push/pull method). Advantages in this case are that no water needs to be fed or exhausted nor remains in the duct.